This invention relates to photoelectrophoretic imaging systems and more particularly to an imaging ink-coated donor sheet for use in such systems.
Photoelectrophoretic imaging in which electrically photosensitive particles are utilized to produce color images is a wellknown method. This process is described in detail and claimed in U.S. Pat. No. 3,384,565; U.S. Pat. No. 3,384,566; U.S. Pat. No. 3,383,993 and U.S. Pat. No. 3,384,488. In such an imaging system colored light absorbing particles are suspended in a carrier liquid. The suspension is placed between electrodes, one of which is preferably conductive and called the "injecting" electrode and one having an insulating layer on its surface and referred to as the "blocking" electrode, subjected to a potential difference and exposed to an image. As these steps are completed selective particle migration takes place in image configuration providing a visible image at one or both of the electrodes. An essential component of the system is the suspended colored particles which must be electrically photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating electromagnetic radiation through interaction with one of the electrodes. In a monochromatic system a single colored image is produced equivalent to black and white photography. In a poly-chromatic system the images may be produced in natural color by providing a mixture of particles of two or more different colors which are each sensitive to light of a specific wavelength or narrow range of wavelengths. Particles used in this system must have both intense and pure colors and be highly electrically photosensitive.
In the above-described process the pigment particles are suspended in a carrier liquid and the suspension is coated on one of the electrodes before the imaging process takes place.
Coating of a thin liquid layer of the desired thickness from the suspension is difficult. Care must be exercised that excessive evaporation of the carrier liquid does not take place between the coating steps and the imaging steps. Storage of the suspension before use is a problem since the pigments may tend to settle out of the suspension. Also, pigments of different colors may not settle out at equal rates requiring careful stirring or resuspension of the pigments immediately before the coating step. Although when properly carried out, the process including the step of coating of the suspension onto an electrode immediately before imaging, is capable of producing images of excellent quality considerable attention must be directed to ensuring that the pigments are uniformly dispersed throughout the suspension when it is coated on the electrodes.
It would be desirable to have materials for photoelectrophoretic imaging whereby the imaging ink suspension can be prepared, then stored for relatively long periods of time and yet not require any reprocessing prior to subsequent use.